1. Field of the Invention
The present invention relates to a magnetic memory device and a write method of the magnetic memory device and, more particularly, to a magnetic random access memory (MRAM) and a write method of it.
2. Description of the Related Art
Magnetic random access memories (MRAMs) are expected as ultimate memories with nonvolatility, high speed, large capacity, and low voltage driving and developed.
For conventional magnetic random access memories, however, it is becoming clear that the capacity can hardly be increased because the write current value increases as the circuit is micropatterned, and the write margin which is small inherently further decreases as the circuit is micropatterned.
To increase the write margin, a magnetic random access memory using toggle switching is proposed (e.g., M. Durlam, et al M,. IEDM Tech. Dig., pp. 995–997, 2003). In this toggle switching method, a synthetic recording layer which almost always has an anti-parallel magnetization arrangement is used as the recording layer of a magnetic tunnel junction (MTJ) element. The write margin is increased by using the spin flop phenomenon (A and B in FIG. 21).
However, according to the toggle switching method, when the short length of the MTJ element is 400 nm, the write current value increases by about 10 mA even when an efficient yoke wiring coated with a magnetic thin film is used.
In micropatterning, to ensure nonvolatility, the decrease in volume of memory cells must be compensated for by increasing the memory energy per unit volume. This results in an increase in reversal field of the recording layer of the MTJ element. As a result, the write current value further increases. When the short length of the MTJ element is about 150 nm, the necessary write current value exceeds the limit of electromigration of the wiring. Hence, a breakthrough is necessary for more advanced micropatterning. Assume that the variation in reversal field largely decreases, and the write current value can be reduced to 4 mA. Even in this case, when the write wiring width is reduced to about 140 nm as the MTJ element is micropatterned, the current density is about 107 A/cm2. This exceeds 106 A/cm2, e.g., the migration tolerance of a Cu wiring. Even in pulse driving, the current density reaches the allowable value limit (the wiring thickness is estimated 280 nm because the aspect ratio is about 2 at maximum in the normal wiring process). Generally, the short length of the MTJ element is set to be almost the same as the write wiring width. Hence, the limit of the short length of the MTJ element is also about 140 nm.
As described above, the most important challenges in the conventional magnetic random access memories are to maintain nonvolatility, reduce the reversal field of the recording layer of the MTJ element, and reduce the write current value to achieve high integration.